You're probably right, they're trying to have the movies stand alone for people who are too lazy to read the books. The problem that I had with this movie is just that a lot of the things that they changed and a couple that they left out, at least in my opinion, would have made it easier for people who haven't read the books to follow what was going on.
It's the same kind of reason why I was pissed that they didn't explain that James was an animagus too, and why he, Sirius and Pettigrew became that way in the first place. EDIT: In the Prisoner of Azkaban movie, I mean.